The atom probe (also referred to as an atom probe microscope or local electrode atom probe) is a device that allows specimens appropriately sized or taken from larger samples, such as semiconductor wafers or large parts thereof, to be analyzed on an atomic level. In a basic version of an atom probe a specimen mount is spaced from a detector, generally a microchannel plate and delay line anode. A specimen is situated in a specimen mount, and the voltage of the specimen holder is adjusted relative the voltage of the detector such that atoms on the specimen's surface ionize, evaporate from the specimen's surface, and travel to the detector. Generally, the voltage of the specimen is pulsed so that the pulses trigger evaporation events with the timing of the pulses, thereby allowing at least a rough determination of the time of evaporation. Measurement of the time of flight of the ionized atoms from the specimen to the detector allows determination of the mass-to-charge ratio of the ions, and thus the identity of the evaporated atoms. Measurement of the location at which the ions impinge on the detector allows determination of the relative locations of the ionized atoms as they existed on the specimen. Thus, over time, one may build a three-dimensional map of the identities and locations of the constituent atoms in a specimen.
Owing to the number of atoms potentially contained in a specimen, and the time required to collect these atoms, specimens are usually taken as a part of a larger object (called the “sample” here). Such specimens are often formed by removing a section or wedge from the sample that represents the structure of the sample throughout at least a portion of its depth. Such a specimen is typically attached to a pre-made post and then sharpened by ion milling. The specimen-post combination is then aligned in a specimen holder with its axis extending toward the detector, so that the collected atoms demonstrate the depthwise structure of the sampled object. The rod-like structure of the prepared specimen also beneficially concentrates the electric field of the charged specimen about its apex (its area closest to the detector), thereby enhancing evaporation from the apex.
Methods in the prior art are directed to the analysis of the atomic structure at the apex of the specimen, as removed from the sample. If the structure of the specimen that is of interest is located at the base of the specimen core, that is, opposite to the apex, there is presently no simple method to quickly prepare that region for analysis in an atom probe instrument. Known methods involve attaching and then detaching a probe tip to the specimen and attaching the specimen to a post. It would be advantageous if the steps of providing a pre-made post and then mounting a specimen upon it by detaching it from a probe tip could be eliminated. The present disclosure solves this problem by making the specimen and the post the same object. Further, prior-art techniques usually require ex-situ deposition of additional layers (such as Pt) to preserve the top layer of the sample from which the specimen is taken, and it would also be advantageous to omit this step if the region of interest is below the surface. Even if it is desired to deposit a protective layer over the sample to protect adjacent regions, it would be preferable to do so in-situ.
In the figures, a scale may be indicated to give the reader the approximate magnification of the configuration depicted. The various dimensional relationships shown are not necessarily exact or to scale.